What it would do is point out members who are helpful and valuable to the community.
That might be what you want it to do, but how do you ensure it does that? That's the key issue.
As others have pointed out, it is possible to game the system. It's possible to game it regardless of the limitations or safeguards you've posted.
Once the system starts being gamed, it loses its value for its stated purpose. In other words, you can no longer trust the Rep number, so you have to look at a person's post history to see if they're trusty or trollish. The post history is the real reputation anyway. That's the content and the context that matters.
Voting on someone else's post history with +1 or -1 (or whatever) is just a context-free McNuggetized opinion.
I dobut those people who act like a 12 year old and show off their reps.
Not any more diferent from post count and I dont see many showing off their post count
Really? Everyone's post count is right there under their name. It's been categorized and named, but it's right there. You're a "macrumors 6502", and I'm a "macrumors 65816". Those names clearly define a range of post counts.
If I want to know an exact post count, that's easily obtainable by clicking on your name in your post. It's also quite easy to see how frequently you're posting (you're averaging 16 a day).
And I can easily see where you've been posting, whether you're asking or answering, and a lot of other things. All available right now, without a rep number.
So really, if I cared about your reputation, I can discover it now, in as much detail as I care to find. And if I don't really care about your reputation as trusty or trollish, then a rep number isn't going to affect anything.